Composition system

ABSTRACT

A composting system having an area for containing a mass of compostable material, and a weatherproof cover. The cover can be conformable to the mass of compostable material and protects the material from environmental factors. The cover defines a composting environment in which environmental conditions may be controlled during the composting process.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/AU96/00771, which has an Internationalfiling date of Nov. 29, 1996, which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

This invention relates to composting systems for treatment of organicwaste in a manner accepted to be environmentally sustainable.

BACKGROUND OF THE INVENTION

Each day a large amount of solid municipal domestic waste is produced.Of all solid municipal domestic waste produced in Australiaapproximately 50-55% consists of food and garden waste. Other componentsof the waste include paper (20%), plastics (6%), glass (10%), metals(5-7%) and other inorganics (10-15%). The organic fraction has a strongdetrimental impact on the environment and may be hazardous.

Detriment results from the large volume of organic waste which mayoccupy 50-70% of landfill space. The waste is of a putrescible nature,thus making it a potential source of pathogenic organisms. A largevolume of greenhouse gases, e.g. carbon dioxide and methane are releasedduring uncontrolled decomposition. Finally, and significantly,decomposing organic matter causes odour, attracts pests and is a majorcontributor to groundwater pollution through dissolution and its role asa carrier or inorganic pollutants such as heavy metals. In certain casessuch pollution may make groundwater unsuitable for safe use.

Therefore, one of the main challenges in any integrated waste managementstrategy is the appropriate and effective treatment of organic waste.The current practice of landfilling organic waste is rapidly becoming aninappropriate waste treatment practice and will be unsustainable in thelong term.

Landfilling consumes large areas of land, results in low land value andis strongly objected to by residents. Consequently, landfilling is fastdisappearing in major cities as a sole waste disposal strategy. Incities with low housing densities, transport costs may becomeprohibitive as suitable landfill space becomes available only well awayfrom waste generation centres.

Organic waste nevertheless has considerable potential as a resource whenstabilised through composting. It is high in organic matter and containsnutrients such as nitrogen (2%), phosphorus (0.5-0.7%), potassium(0.7-1.7%) and trace elements.

Composting is the process whereby organic matter is decomposed by arange of microorganisms using oxygen. The process is appropriate fortreatment of. a combination of fibrous waste (e.g. green organics) andputrescible waste such as food waste, sewage sludge and industrial andcommercial organic residues. Composting has the advantage of reducingthe waste volume by 30-40%. In addition, product compost has significantbenefits as a soil conditioner.

A typical composting process may comprise four stages. Each stage ischaracterised by the activity of different generations of bacteria,fungi, protozoa and actinomycetes. During each stage the microbes useoriginal organic compounds present in the waste as well as by-productsof the metabolism of the previous generation as a nutrient and energysource. Thus the organic matter decomposes until a stable humus isformed.

The incubation or mesophilic phase lasts for approximately 24 hoursduring which the organic matter is rapidly invaded by mesophiliccomposting organisms including bacteria, actinomycetes and fungi. Theseorganisms thrive at a temperature of 25°-45° C. The mesophilic organismsgrow in this phase on the more easily assimilated substances present inthe organic waste, for example: sugars, soluble protein, starch andorganic acids.

The high metabolic activity of the organisms and the exothermicdecomposition processes that result, in combination with the insulatingproperties of the composting material, causes the temperature to rise.The temperature rise strongly favours thermophilic sporogenous bacteria.The activity of these bacteria takes the process into the thermophilicphase.

During the thermophilic phase, organic matter is decomposed rapidly.Temperatures may reach 70° C. in the core of the composting material.This is undesirable, since at this temperature most processparticipating microbes, including some thermophilics, are killed. Thismay considerably reduce the decomposition rate of organic material.About 45°-50° C. is optimum and above 55° C. is typically required forpathogen destruction, thus 55° C. is accepted as an optimum temperaturecompromising between these factors, at which the decomposition rate ishighest. These temperatures assist in accelerating the process andsanitising the material from pathogens, weed seeds and plant diseasecausing agents. This temperature, and below, allows the development ofeumycetes and actinomycetes which are the main decomposers of long chainpolymers, cellulose and lignin. The oxygen demand is very high in thisphase and aeration is required. This phase may last for 2-3 weeksdepending on aeration and substrate.

The cooling phase commences when there is insufficient exothermicorganic substrate left to maintain the high temperature. Accordingly,water evaporation and heat convection cause temperature to drop. If thetemperature drops below 45° C. mesophilic bacteria and other organismsmay reinvade the fresh compost. This phase may last a few days.

A maturation or stabilisation phase is required to allow the toxicity offresh compost to fall to enable effective utilisation by plants. Theactivity of fungi, protozoa and actinomycetes may be highest during thisphase, while bacterial activity slowly falls. At this stage, largepolymers such as lignin and cellulose are decomposed and ahumidification process sets in. The activity of actinomycetes producesthe compound “geosmine” which gives matured compost a fresh earthysmell. Three to four weeks may be sufficient to enable completion ofthis phase.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a compostingsystem that may enable the respective phases of the composting processto be completed in an efficient manner, under conditions that enable themost advantageous conditions for aerobic microbial decomposition of anorganic substrate to occur.

With this objective in view, the present invention provides a compostingsystem comprising an area for holding compostable material; a mass ofcompostable material located in the area; the area and a weatherproofcover or structure for the area defining an environment in whichcomposting occurs, and an environmental condition which is controlled tooptimise the composting process.

In particular, the weatherproof cover is preferably to be secured orappropriately sealed to prevent ingress of water or other environmental,especially climatic, impacts detrimental to efficient composting such asexcessive drying out. Entry of pests and vermin is also prevented. Aflexible or modular construction for the cover is preferred,particularly a cover which is readily conformable to the volume ofmaterial to be composted. The volume of the environment is dictatedprimarily by the volume of compostable material to be treated.

In this respect, the invention is predicated on the discovery that thecomposting process is dependent upon a number of variables, the controlof which ensures a more efficient composting process. Further, the coverprevents escape of odour, water ingress and generation of leachate.Leachate generation is a particular problem in open air compostingsystems such as windrow composting and static pile forced aerationcomposting which are strongly dependent on weather conditions. Whererainfall is high, water may drain through compost windrows, leachingnutrients and soluble organic matter from the compost. The generatedleachate may usually require treatment before discharge to surface orground water and compost quality is reduced.

For a first example, the control of aeration may be important toconducting the composting process. Aeration provides the oxygennecessary to sustain the aerobic organisms that promote composting. In astatic pile, oxygen levels can drop to below 1% by volume and carbondioxide levels can reach 20% by volume. Such levels may be inhibitive tocomposting.

Therefore, aeration means to aerate the mass of compostable material areto be included within the composting system. The aeration means mayprovide a variable controllable proportion of recycled process air andfresh air assisting in maintenance of the compostable mass moisture atdesired levels importantly preventing drying out of compost, and mayprovide oxygen levels within the mass of 10-18% by volume.

Control over the level of carbon dioxide in the mass of compostablematerial may also be desirable. In this respect, air flow may becontrolled such that carbon dioxide levels are maintained below 10% byvolume.

In this respect, the O₂ and CO₂ levels are interlinked and add up to±21%. Thus if oxygen is 15%, carbon dioxide is 6%.

Accordingly, the composting system may advantageously include means formaintaining an appropriate moisture level. In this respect,recirculation of spent process air or oxygen through the mass ofcompostable material may be conducted to cause a flow of moist air whichmaintains moisture in the mass at desired levels and a carbon dioxidelevel of approximately 15% by volume. Recirculation of air or aerationby other means may also assist in achievement of a homogeneous moisturelevel throughout the mass avoiding stratification or formation of dryspots which adversely affect composting. Fresh air may then beintroduced by blower or other air compressing means to maintain a ratioof recycled air to fresh air, sufficient to maintain the desired oxygencarbon dioxide level. Alternatively, fresh air or oxygen may beintroduced at any time to maintain a desired recycle air to fresh airratio and/or carbon dioxide concentration.

This ratio may be maintained at the desired level in dependence uponmonitored oxygen or carbon dioxide level but may also be controlled as afunction of other composting process variables, such as the temperatureof the core of the mass of compostable material. Suitable sensors may beprovided for these purposes and the composting system may be under thecontrol of a microprocessor or like device.

Where warm, moist air contained within the cover is mixed with fresh airand recirculated through a blower or like means, the temperature andmoisture level of the air entering the core of the compost windrowincreases preventing drying out and/or premature cooling.

The weatherproof cover may be sealed at its edge by a low cost sealingmeans such as sandbags, soil, a water jacket, beams rods or other means.It is important that the sealing be achieved in a manner that enableseffective control over the microenvironment within the weatherproofcover. Space may be at a premium. In this respect, the system is ideallydesigned to exclude climatic influences over the composting process,chiefly drying influences and excess rainfall which may base excessmoisture and leachate generation or prolonged hot and dry conditionswhich may dry the compost to a point where microbiological activityceases. The system is also advantageously flexible to suit variation inthe mass of material to be treated and sealing means may be selectedwith this in view.

A clear manifestation of climatic influence is rainfall. Rainfall maymarkedly interfere with a composting process because the level ofmoisture in the mass of compostable material is an important processparameter. While the composting microbes may require a minimum moisturecontent of about 40% to avoid reduced activity, levels of moisture aboveabout 60% may lead to the occurrence of anaerobic conditions whichchange the process from a composting process to a fermentation (orrotting) process. This will occur when the pores in the substrate fillwith water to an extent that effective aeration is impaired. Further,excess water may be a cause of leachate generation, undesirable forenvironmental reasons, for example, base metal pollution anduncontrolled nutrient loss to the environment. Hence, the weatherproofcover is ideally to be a material that prevents ingress of water to thecomposting system due to rainfall and which prevents excess moistureloss due to drying, especially of the edges, during hot weather.Ideally, the material should facilitate collection of moist spent airand dry fresh air.

In addition, temperature may be important. Composting involves athermophilic stage and drop in ambient temperature may effect this. Thusthe material may be of a kind that prevents ingress of ambient air tothe mass of compostable material and escape of odour. If necessary,provision for addition of make up fresh air may be made to minimise theeffects of ambient temperature.

Typically, the system through controlled aeration and exclusion of waterenables compost to be held at a sufficient temperature, defined by somestandards as 55° C. or higher, for a sufficient period, a few, typicallythree, days to kill pathogenic organisms present in the material orbiosolids.

Pathogens, both plant and human, are inherent to most types of organicwastes. In order to minimise risks to public health and flora, i.e.crops, such materials must typically be processed such thatsubstantially complete pathogen destruction is achieved.

The provision of air circulation through the compostable material mayassist in avoiding a situation where low temperature zones are formed atthe base of a pile or a periphery of a windrow where excessive heat lossto the atmosphere and lack of insulation may prohibit temperaturesreaching thermophilic levels. Drying out may also occur at thisperiphery in open windows.

Product compost which may typically have less than ten (10) totalcoliforms per gram compost in comparison with ARMCANZ guidelines (seeAgriculture and Resource Management Council of Australia and New ZealandWater Technology Committee, Guidelines for Sewage Systems—BiosolidsManagement Occasional Paper WTC No 1/95 October, 1995) which specifythat Class A compost may contain a maximum pathogen concentration of 100thermotolerant coliforms per gram compost.

Typically, the compostable material may be turned once during theprocess (duration usually approximately 8 weeks) although turning may beconducted more frequently. However, frequent turning is undesirable aslabour and equipment costs may be increased and it is not an aspect of apreferred embodiment of the present invention.

In a further aspect, the present invention provides a composting methodcomprising delivering a mass of compostable material to an area forholding compostable material.

The area having a weather proof cover or structure for the area definingan environment in which composting occurs, an environmental condition ofwhich is controlled to promote the composting process.

The composting system and method of the present invention may provide anumber of advantages. Firstly, the system allows control over odour, the“balloon” formed by the preferably flexible weather proof cover aroundthe mass of the compostable material including biosolids preventingodour emission. Recycled air may be deodorised by the compost massacting as a biofilter and excess air which requires venting may be ledthrough a biofilter for substantially complete odour removal. Leachatesare not generated in any significant amounts and may be contained andnot released to the surrounding environment, at least in an untreatedstate. In addition, the “balloon” creates a homogeneous microenvironmentwhich is controllable to the benefit of efficient composting. Thecontrol achieved over moisture level and pathogens may allow, forexample, a more rapid composting rate. In addition, the system offers abenefit of low cost with the various components available in mostlocations at low cost. Low cost also offers the advantage of plantmobility with low capital risk when the system is moved from place toplace.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more fully understood from the followingdescription of a non-limiting embodiment thereof made with reference tothe accompanying drawings in which:

FIG. 1 is a transverse section through a composting system in accordancewith one embodiment of the invention;

FIG. 2 is a plan view of the composting system in accordance with theembodiment of FIG. 1;

FIG. 3 is a temperature profile of a region of the system constructed inone embodiment of the present invention; and

FIG. 4 shows a transverse section of a second preferred embodiment ofthe composting system of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a transverse section of thecomposting system which comprises an area or base 1 on which is piled amass of compostable material 2. The compostable material comprisesorganic waste, for example from domestic sources, biosolids, thoughother sources of waste, such as abattoirs, may be available. It may benecessary to separate the organic waste or green waste from a mixedsolid waste containing paper, glass, metals, plastics and other refuseby appropriate steps, for example a system of mechanical sorters, airclassifiers, magnetic conveyor belts and the like. The material mayrequire particle size reduction by means of shredding or other forms ofcomminution, for example in mills (usually hammer mills) to obtainoptimum particle size from the point of view of porosity structure andspecific surface area.

Heavy metal analysis for acceptability may also be conducted prior totreatment commencement. If desired, an adsorbent for heavy metals, suchas bauxite processing residue, may be added in accordance with themethod of applicant's Australian Patent No. 661703, the contents ofwhich are hereby incorporated by reference. Non-organic waste mayaccount for 10-20% of the total mass of waste. Any putrescible waste maybe delivered to area 1, if desired, as an amendment to the organic greenwaste. Moisture content of the compostable material 2 may be controlledto prevent drainage of excess liquid from the material. Any excessprocess water may condense against the inside of cover 7 and drainsideways for collection by discharge trench 3 or like means. Thiseffectively distilled water may be collected in a ground collection tankor similar means preferably to be used to humidify any influent air tothe process.

The area 1 comprises a solid base, made for example, from concrete,brick, compacted limestone or other possibly impermeable material whichprovides an all weather working surface for the mass of compostablematerial 2 and ideally appropriate insulation to the climaticenvironment. However, as leachate is not generated in significantquantities it is not essential that the base be water impermeable. Thebase may be modular and portable and may also advantageously insulatethe mass 2 from surrounding ground advantageously avoiding of lowtemperature regions at the bottom of the mass. Such low temperatureregions do not compost as desired and pathogen levels may remain high.Indeed, temperature is relatively homogeneous allowing composting andentry to thermophilic phase by a substantial portion of the mass ofcompostable material, resulting in general pathogen destruction. Thebase, together with the cover 7 define the composting environment, theenvironment has a volume sufficient to enable efficient composting ofthe mass of compostable material and, typically, the cover 7 will be inclose proximity to the mass of compostable material. It is not desirablethat the non-composting volume be at all significant as this maydetrimentally affect composting process control.

At the bottom of the mass 2 is an air inlet or aeration trench 3 whichforms one portion of the means to aerate the mass 2. Air is circulatedthrough the trench 3 by means of a blower 4, shown in plan view in FIG.2. The trench 3 is of approximately rectangular section though the ditchis not required to be restricted to rectangular geometry. The geometrycould readily be semi-circular, ovoid or any other convenient geometry.Similarly, the trench 3 might be replaced with another kind of aerationmeans, located anywhere beneath the surface of the mass, for example theaeration means could take the form of a ditch or perforated tubing. Anumber of ditches, tubes or trenches could be employed, above or belowground surface level.

At the top of the ditch 3 is a grid 5, optionally made from a metallicmesh. The mesh is ideally constructed of a corrosion resistant material,such as stainless steel, to resist the corrosive influence of moist airfrom the mass 2.

The grid 5 serves two purposes. Firstly, the grid 5 prevents subsidenceof compostable material into the ditch 3 which would reduce theeffectiveness of aeration by blocking the air supply to the compostablematerial. Further, the apertures of the grid 5 tend to distribute theflow of air so as to better aerate the compostable material. Trench 3 oralternative means of similar function may also serve as a collectiondevice for condensed water allowing recovery, preferably for use inhumidifying influent air to the process.

At the edge of the area 1 is a gutter 6 that defines the perimeter ofthe area for holding compostable material. As well as receivingrainwater and allowing diversion of water away from the compostingsystem, the gutter 6 provides a convenient location for sealing of theweatherproof cover 7 that surrounds the mass of compostable material 2.Rainfall may run off cover 7 and may be directed to flow into the gutter6 which may be permitted to run into a usual stormwater dischargesystem.

As observed from FIG. 1, the mass of compostable material 2 forms alongitudinal approximately triangular pile to which the cover 7, beingof a flexible low cost material, is conformed to create amicroenvironment within the cover 7 in which composting can take place.

The cover 7 is conveniently of a low cost weatherproof material, such ashigh density polyethylene (HDPE) or PVC, woven or in the form of a film,though other polymers may also be suitable, that is waterproof anddurable to prevent entry by vermin and insects, ingress of water due torainfall which may cause a leachate flow to ground 11 surrounding area1. The cover 7 also excludes undesirable materials and chemicals such asweed seeds and pesticides from contaminating the compost. It is alsoadvantageously robust to other climatic influences such as windy and dryconditions which may cause drying out of the mass and a fall incomposting rate to unacceptably low levels. Ideally, the cover 7 may beof air tight material to prevent ingress of air and prevent escape ofodour and moisture caused by formation of volatile organic compounds andammonia during the composting process. The cover 7 may be made of areasufficient to adapt to varying mass and volume of compostable materialand appropriate folding in the region of edge 8 to achieve the desiredarea may be undertaken for this purpose. Flexibility or modularconstruction to allow for varying volumes of compostable material mayenhance this advantage. The cover 7 itself could be provided withsealing means, for example, water pockets, as described below.

The edge 8 of the cover is sealed against ingress of water and escape ofodour by a water jacket 9 above the edge 8 of the cover preferablyforming an air-tight seal for the latter purpose. The volume of waterimposes sufficient force to maintain a seal, though other means forachieving a seal may of course be adopted. For example, sufficientlyheavy objects such as beams, rods, sandbags or soil may be laid in thegutter 6 to achieve the same end. Alternatively, sealing may be achievedby straps extending over the cover 7 and surface area to enablesecurement and sealing. Simple sealing means facilitate the conformanceof the composting environment to varying volumes of compostable materialcontributing to system flexibility.

The aeration means is completed by the air recycle pipe 10 which takesspent process air away from the mass of compostable material 2 under theinfluence of blower 4 by suction. The pipe 10 may be simply perforatedor agricultural tubing which collects process air and recycles it to theblower 4 which, in a preferred embodiment as shown in FIG. 1, has asubstantial portion thereof provided along the bottom edge(s), one beingshown in plan view in FIG. 2, of the mass of compostable material, andwhich takes circulated air away from the mass of compostable material 2under the influence of blower 4. Alternatively, as shown in FIG. 4, asubstantial portion of the recycle pipe 10 may be provided at the top ofthe system, the flexible pipe being fixed by a portion of the cover 7bridging section 7 a and 7 b thereof by suitable fastening means.

Turning now to the plan view of the composting system shown in FIG. 2 itmay be seen that air inlet trench 3 extends longitudinally along thebase of the mass of compostable material 2 preferably substantially thewhole length thereof.

The air passing through the air recycle pipe 10 is returned to theblower 4 and, upstream thereof may be located a moisture, oxygen orcarbon dioxide concentration sensor 11 which monitors the concentrationof one or other of the gases. In accordance with the monitored gasconcentration, a valve or other means, as understood in the art ofblowers, may be set either manually or automatically to proportion freshair, by means of blower 4, into the air inlet ditch 3 to achieve thedesired levels of moisture, oxygen and carbon dioxide through, controlvia the blower and proportioning means, an appropriate ratio of recycledair to fresh air. If a valve is used, the valve may be of solenoid orother suitable type as may be understood by those involved inengineering of pneumatic systems. The recycle/fresh air mixture passesthrough pipe 13 to air inlet ditch 3. pipe 13 may be of any convenientplastic material, e.g. PVC. Fresh air addition assists in maintainingpreferred composting conditions. Any air discharged to maintain thebalance of fresh to recycled air may be treated, for example, by asuitable biofilter to remove odour forming compounds.

The rate of aeration of the mass of compostable material of the mass ofcompostable material 2 may also be controlled in accordance with thetemperature sensed by temperature sensors located at desired locationswithin the system. The most advantageous locations for temperaturesensors are at the air inlet, the core and in the surface regions of themass of compostable material 2. Temperature. at these locationsgenerally reflects the efficiency of the composting process.

Spent process air recycling may serve an important role in reducingodour as the odour causing compounds may typically be adsorbed by thecompostable material which may act as a biofilter after recycling.

The system may include a covered raw material storage area for materialawaiting pretreatment and conditioning prior to being placed in thesystem and air from this area may be extracted from this area by blower4 for use in aeration of the windrow or otherwise treated for odourremoval. Thus the potentially odorous air from the raw material storagearea may be vented to the system accordingly, further assisting in odouremission control.

The system may be conveniently placed under manual or automatic control,for example, of an electronic control unit. A computer control system isdesirable for this purpose. In accordance with measured variables, suchas temperature, O₂ concentration, CO₂ concentration, moisture level, theblower 4 may be operated at a desired recycle air: fresh air ratio tomaintain composting conditions at an optimal level from the point ofview of microbiological activity, and composting may proceed with littleintervention from personnel.

Composting should continue for sufficient duration to diminish thelevels of phytotoxic compounds caused by intermediate metabolites andhigh ammonia levels in immature compost. Ideally composting shouldproceed at least three weeks and preferably eight weeks per tonne ofwaste.

The contained nature of the system allows stable composting undercontrolled conditions. The final product is more acceptable from thestandpoint of both environmental and commercial considerations and maybe implemented at relatively low cost with reduced processing time.

The present invention may be particularly advantageous where medium tolarge quantities of waste need to be processed in environments whereodour emissions and leachates are of concern. Therefore, residentialareas, such as cities, may be areas of typical application.

There will now be described the performance of a development systemoperated in accordance with the system and method of the presentinvention as described above with particular reference to the preferredembodiment. In this example, the effectiveness of the system inprocessing a mixture of green waste and biosolids, in particular sewagesludge (anaerobically digested primary sludge, not typically treated incomposting), was evaluated.

Process Heat Distribution

The ARMCANZ guidelines (see Agriculture and Resource Management Councilof Australia and New Zealand Water Technology Committee, Guidelines forSewage Systems—Biosolids Management Occasional Paper WTC No 1/95October, 1995) require composting to be carried out at thermophilictemperatures (>55°C.) for at least three continuous days to producegrade 1A compost. The ARMCANZ grade 1A composting requirements were metthroughout the windrow using the method and system of the invention,including at the windrow surface. Reference is made to a FIG. 3 whichshows a temperature profile at the windrow surface and various depthswithin the pile. It has been found in previous practice that in openwindrow composting systems the surface temperatures are too low (nearambient, at 15-30° C.) to meet these guidelines. It is thought, withoutwishing to be bound by any theory, that the preferably flexible cover ofthe system insulates the windrow from climatic conditions therebypreventing heat loss. Materials for the cover may be selected with thisin view. Such a cover may also trap solar radiation which may assist inheating the windrow surface. It has been found that windrow surfacetemperatures may reach up to 70° C. during composting and this a specialadvantage of the present system. An air layer containing trapped airwill typically be present and acids in insulation of the compost mass.

The thermophilic stage may be extended by including “thermophilic”amendments such as sawdust or wastewater skimmings to the compost.Without wishing to be bound by any theory, skimmings, chaff or materialsproviding more energy per mole of carbon than sawdust may be preferredas an amendment though sawdust may be used. The lipid content ofskimmings is a factor in providing a higher energy output.

It will be noted that in forced aeration composting systems known to theprior art, the windrow core is often subjected to excessive evaporativecooling such that the above guidelines may not be met. In the presentsystem, this problem may be alleviated by regulating aeration with acomputer controlled system to satisfy the composting aerationrequirements without excessive cooling. Further, recycling of moist andwarm windrow exhaust air back into the system may further reduceevaporative cooling.

Pathogen Levels

It was found that pathogen levels in compost made by biosolids/greenwaste using the present system complied with the above guidelines forcompost of the highest standard, grade 1A compost. MPN <2 is onlylimited by the detection limit of the methodology. Therefore productcompost from the system was found to be suitable for unrestricteddistribution. In an open windrow system, pathogen reinfestation ofsanitised material from inside the windrow may occur at or after turningby mixing of the sanitised material with the pathogen infested surfacematerial or and/or bringing the sanitised material from inside thewindrow to the surface thus exposing the material to vectors likeinsects and vermin.

TABLE 1 Pathogen Levels in the Final Biosolids Compost (82 days old)Compared to Grade 1A Biosolids. Pathogens in ADPS compost (MPN) Grade 1AGrade 1B Sample location Pathogen (MPN) (MPN) 1 2 3 4 Salmonella <1 <10<2 <2 <2 <2 (per 50 g) Thermotolerant <100 <1000 43 23 9 <3 coliforms(per g) NB 1 = surface; 2 = surface; 3 = 1 m below surface; 4 = 15 cmbelow surface

It may be seen from Table 1 the product compost complies with ARMCANZguidelines for grade 1A compost.

Leachate Generation

The system of the invention is designed to prevent generation ofleachate during processing or composting. This is one of the benefits ofthe system. The ground directly beneath the windrow was tested forleachate. Ammonia nitrogen (found in higher concentrations in thewindrow material) was used as a tracer to indicate leaching. The(compost) concentration of ammonia nitrogen in 1/5 (w/v) water extractsof the windrow material was 28 ppm compared to the soil ambient level ofbetween 1 and 2 ppm. The ammonia nitrogen levels of the soil 15 cm belowthe ground surface under the windrow were at ambient levels, i.e. at 1to 2 ppm. Therefore it was found in the experiment that the transfer ofammonia nitrogen (hence leachate) from the windrow material into theground did not occur and this lack of leachate percolation wasadvantageous.

Odour

This was assessed qualitively and only a slight smell could be detectedby persons located at the composting site, the smell being described asa “sweet” compost smell. None described the odour as unpleasant, as withpure untreated biosolids. The flexible plastic cover may provide a sealagainst emission odours which would otherwise have been much higher andthis may be advantage of the system.

General Compost Quality

This is most important to the marketability of compost especially as asoil amendment. Product complied with Standards Australia Draft StandardDR 95301. Quality parameters of produced compost in the experiment areshown in Table 2.

TABLE 2 General Composting Parameters of the Biosolids/Green WasteMixture. Final Product Parameter Initial Material (8 weeks) Totalorganic carbon, % 31.1 28.7 Total organic nitrogen, % 1.67 1.95 Carbonto nitrogen ratio 18.6 14.7 Total phosphorus 0.61 0.72 Buffer capacity,% CaCO₃ 16.7 21.1 Ash, % 40.1 50.7 Salinity (Elec conductivity in mS)2.37 2.46 pH 7.6 6.7

Typically composting increases quality of material by concentratingnitrogen and phosphorus. The C/N ratio typically suitable for landapplication after composting. Composting typically increases ash contentas it is expected to remove the readily putrescible organic matter whichmay encourage pathogen regrowth. It was found that the electricalconductivity (salinity) and pH of compost were ideal for landapplication.

It will be understood that modifications to the above describedcomposting system may be made without departing from the scope of thepresent invention.

In particular, the system need not be controlled identically with thedescription above. Furthermore, the system may comprise an area withother than approximately rectangular geometry.

The air could be treated for odour removal, for example by treatmentwith an adsorbent prior to recirculation if adsorbent capacity of thecompostable material is insufficient.

Further, a full scale plant might comprise a number of, preferablyparallel covered windows with air recirculation possibly being under thecontrol of an integrated control system.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

The claims defining the invention are as follows:
 1. A method ofoperating an open air composting system including (a) an area having abase for holding and treating compostable material; (b) in use, a massof compostable material located in the area; (c) a weatherproof coverhaving an outer surface which conforms during composting, to the mass ofcompostable material and being secured to said base of said area fordefining with said base a composting environment; (d) means for securingthe cover to the base for forming said composting environment; (e)aerating means including a gas inlet means for flowing moist oxygencontaining gas to said composting environment and gas outlet means forcollecting moist spent process gas from said composting environment, aportion of said moist spent process gas being recycled to the compostingenvironment in combination with a portion of fresh oxygen containinggas; and (f) a control unit for controlling an operation of the aeratingmeans during composting wherein a ratio of said portion of moist spentprocess gas recycled to said composting environment to said portion offresh oxygen containing gas is controlled in accordance with a measuredvariable of said portion of said spent process gas recycled to saidcomposting environment to control oxygen concentration and moisturecontent of said mass of compostable material, said method comprisingdelivering the mass of compostable material to the area for holdingcompostable material, the area and the weatherproof cover or structurefor said area defining an environment in which composting occurs, anenvironment condition of which is controlled to promote the compostingprocess.
 2. The method of claim 1 wherein said control unit controls amoisture content in the process air.
 3. The method of claim 1 whereinthe environment is substantially filled with compostable materialleaving an insulating layer of air between the material and the cover.4. The method of claim 1 wherein the system includes a covered rawmaterial storage area vented for supplying air for aeration of said massof compostable material.
 5. The method of claim 1 wherein spent processair is recovered by a recycle pipe, a substantial portion of saidrecycle pipe being located above said compostable material at the top ofsaid cover.
 6. The method of claim 1 wherein spent process air isrecovered by a recycle pipe, a substantial portion of said recycle pipebeing provided along bottom edges of said mass of compostable material.7. The method of claim 1 wherein an absorbent is added as an amendmentto said mass of compostable material.
 8. The method of claim 1, whereineither of a bauxite process residue or an absorbent is added as anamendment to sail mass of compostable material.
 9. An open air methodfor composting a mass of compostable material comprising: (a) forming amass of compostable material in an area having a base for holding saidmaterial; (b) conforming a weatherproof cover to said mass ofcompostable material for defining with said base an enclosed compostingenvironment on securement to the base; (c) securing the cover to saidbase to form said composting environment; (d) flowing a moist oxygencontaining gas through said mass of compostable material duringcomposting by an aerating means including a blower and gas inlet meansfor flowing said moist oxygen containing gas to said enclosed compostingenvironment and gas outlet means for collecting moist spent process gasfrom said composting environment; (e) recycling a portion of said moistspent process gas to said composting environment in combination with aportion of fresh oxygen containing gas; and (f) controlling flow of themoist oxygen containing gas through the mass of compostable materialduring composting with a control unit wherein a ratio of said portion ofmoist spent process gas recycled to said composting environment to saidportion of fresh oxygen containing gas is controlled in accordance witha measured variable of said portion of said spent process gas recycledto said composting enviroment to control oxygen concentration andmoisture content of said mass of compostable material.
 10. The method ofclaim 9 wherein an aerating means provides a flow of process air throughsaid environment.
 11. The method of claim 10 wherein fresh air is addedto the flow of process air delivered by the aerating means.
 12. Themethod of claim 9 wherein air is discharged through the environmentfollowing treatment for odor removal.
 13. The method of claim 9 whereinprocess water generated within the environment is employed in control ofmoisture levels in air delivered by an aerating means.
 14. The method ofclaim 9 wherein spent process air is recovered by a recycle pipe andreturned to a blower.
 15. The method of claim 9 comprising the furtherstep of separating organic waste from a mixed solid waste prior toforming said mass of compostable material.
 16. The method of claim 9,wherein the step of securing said removable cover to said base includesthe step of forming a substantially air-tight seal between the removablecover and the base.
 17. The method of claim 9, wherein the step ofconforming a removable cover to said mass of compostable materialincludes the step of substantially conforming a shape of the removablecover with an exterior of the mass of compostable material.
 18. An openair method for composting a mass of compostable material comprising: (a)forming a mass of compostable material in an area having a base forholding the mass of compostable material; (b) conforming a weatherproofcover to the mass of compostable material for defining with the base asealed composting environment with an outer surface of the weatherproofcover is secured to the base; (c) securing the cover to the base to formthe sealed composting environment; (d) flowing a moist oxygen containinggas through a portion of an aerating means located in the base andthrough said mass of compostable material during composting, saidaerating means also including a blower and gas inlet means for flowingsaid moist oxygen containing gas to said enclosed composting environmentand gas outlet means for collecting moist spent process gas from saidcomposting environment; (e) recycling a portion of said moist spentprocess gas to said composting in combination with a fresh oxygencontaining gas; and (f) controlling flow of said moist containing gasthrough the mass of compostable material during composting with acontrol unit wherein a ratio of said portion of moist spent process gasrecycled to said composting environment to said portion of fresh oxygencontaining gas is controlled in accordance with a measured variable ofsaid portion of said spent process gas recycled to said compostingenvironment to control oxygen concentration and moisture content of saidmass of compostable material.
 19. An open air method for composting amass of compostable material comprising: (a) forming a mass ofcompostable material in an area having a base for holding said material;(b) conforming a weatherproof cover to said mass of compostable materialfor defining with said base an enclosed composting environment onsecurement to the base; (c) securing the cover to said base to form saidcomposting environment; (d) flowing a moist oxygen containing gasthrough said mass of compostable material during composting by anaerating means including a blower and gas inlet means for flowing saidmoist oxygen containing gas to said enclosed composting environment andgas outlet means for collecting moist spent process gas from saidcomposting environment; (e) recycling a portion of said moist spentprocess gas to said composting environment in combination with a portionof fresh oxygen containing gas; and (f) controlling flow of said moistoxygen containing gas through the mass of compostable material duringcomposting with a control unit wherein a ratio of said portion of moistspent process gas recycled to said composting environment to saidportion of fresh oxygen containing gas is controlled in accordance witha measured variable of said composting environment to control oxygenconcentration and moisture content of said mass of compostable material.